Paper machinery



July 1, 1969 J. E. PENKALA ETAL. 3,45 ,137

PAPER MACHINERY z? of 2 Sheet FIG-4 Flled Oct FIG-3 FIG-8 FIG-7 FIG-9 I25 Q &a v INVENTORS 1 as JOSEPH E. PENKALA a Q i BY JEAN-PIERRE ESCARFAIL I37 7 My ATTORN United States Patent 3,453,137 PAPER MACHINERY Joseph E. Penkala, Fulton, N.Y., and Jean-Pierre Escarfail, Pontcharra, France, assignors to The Black Clawson Company, Hamilton, Ohio, a corporation of Ohio Filed Oct. 5, 1965, Ser. No. 493,027 Int. Cl. Bc 11/02 US. Cl. 117-102 13 Claims ABSTRACT OF THE DISCLOSURE This invention relates to the art of applying a coating to a smooth surface, and particularly to a process and apparatus for applying a coating to a traveling web.

At the present time the speed of machinery for applying a coating to a web of paper impervious film is limited by the coater or fluid applicator, rather than the drives, driers, and unwind and rewind equipment. One possible exception is the blade coater which uses a flexible 'blade pressed against the traveling web. However, this type of machinery cannot produce a wet film wherein a portion of the coating remains above the surface of the web since the flexible blade actually contacts the web. Consequently, the machinery cannot be used to apply a coating to an impervious web, e.g., plastic material.

It is difficult to obtain a speed above 1000 feet per minute with existing wet film coaters due primarily to .the use of a diverging nip area in most of these coaters. Diverging nips allow rapid release of internal pressure in the coating that eventually breaks the wet film or produces a pattern even before a reasonably high speed can be developed. While the air doctor type of wet coater may not suffer from the problems of a diverging nip, it does experience coating aspiration which becomes acute at pressures over 3 p.s.i. It is therefore impossible to obtain a high quality coating at high speeds and during long runs with the air doctor.

Although reverse roll coaters can be operated at high speed, high quality results thereon require high tolerances of the rolls. Thus the rolls have as much as One-tenth mil variation between the cooperating rolls, the coating thickness can vary one-half .to three-quarter pound in a coat weight of five-ten pounds.

Accordingly, an important object of this invention is to provide a process and apparatus for applying a wet coating to a web of paper or other material at high speed and with high quality results.

Another object of this invention is to provide a process and apparatus for doctoring the coating on a web which permits a wide range of coating thicknesses without creating undesirable patterns in the coating, and particularly to provide such doctor apparatus which can be used with substantially any type of apparatus for applying the coating to the web.

A further object of this invention is to provide apparatus for doctoring the coating on a web moving at high speed wherein the coating itself cooperates with the doctor apparatus to smooth and even the coating and to remove a portion thereof to achieve a coating of desired thickness, and further to provide a doctor apparatus which 3,453,137 Patented July 1, 1969 operates equally as Well over a wide range of web speeds and wherein the pressures generated in the coating are not sufiicient to cause detrimental flow into the substrate of the web being coated.

Another object of this invention is to provide apparatus for doctoring a coating which is simple in design and is self-tilting to accommodate automatically the speed of the web and the viscosity of the coating to produce a coating of uniform .thickness and free of patterns, and particularly to provide a doctor apparatus of the aforesaid type which can be used with a fountain type coater or a reverse roll coater, as well as to coat simultaneously both sides of a moving web.

Other objects and advantages of the invention will be apparent from the following description, the accompanying drawings and the appended claims.

In the drawings:

. FIG. 1 is a side elevation, partially in section, of the coating apparatus in accordance with the invention;

FIG. 2 is a plan view of the coating apparatus shown in FIG. 1;

FIG. 3 is an enlarged side elevation showing the doctor bar and its supporting structure;

FIG. 4 is a schematic illustration which demonstrates the dimensional consideration in the doctor bar;

FIG. 5 is a side elevation view of another form of the invention;

FIG. 6 is an enlarged side elevation view of the fountain coater embodiment shown in FIG. 5;

FIG. 7 is a side elevation view of a reverse roll coater embodiment of the invention;

FIG. 8 is an enlarged side view of the embodiments shown in FIG. 7; and

FIG. 9 is a schematic side elevation view of another embodiment of the invention wherein both sides of the web are coated simultaneously.

Referring to the drawings, which illustrate preferred embodiments of the invention, FIGS. 1 and 2 show a portion of the frame 10 of a coater which includes a pair of spaced end stands 12 supporting a backing roll 11. Suitable frame and end stand structure for this purpose are shown in the United States patent of Jacobs et al. No. 3,079,889, issued Mar. 5, 1963, and assigned to the as -signee of this application.

FIG. 1 further illustrates the coater apparatus including a fountain coater assembly 13 which applies the coating to one surface of a web 14, and a doctor assembly 15 for smoothing and removing any excess coating applied by the fountain. It is this latter assembly to which this invention is primarily directed, and the fountain coater assembly which is shown for purposes of illustration is described and claimed in the copending United States application of Richard W. Phelps et al., Ser. No. 408,163, filed Nov. 2, 1964, and assigned to the assignee of this application.

The fountain assembly 13 includes a conduit or tube 16 which extends the width of the roll 11. The tube 16 is closed or blocked at the ends thereof and a coating liquid is supplied to the interior of the tube through the flexible inlet conduit 18, which connects centrally to the tube 16.

The fountain assembly 13 further includes a fountain bar 20 mounted on an upper surface of the tube 16 and also extends the width of the roll 11. The bar 20 is formed with a longitudinal groove 22 which may be rectangular in section, and this groove 22 opens into the interior of the fountain tube 16 through a series of longitudinally spaced openings 23 in the bottom of the bar 20 and the top of the tube 16. The bar 20 and the tube 16 may be held together as a unitary assembly by welding to the tube 16 or any other conventional manner. The fountain bar 20 is formed with the upstream and downstream longitudinal lips 25 and 26, each of which has a flat surface adjacent the roll 11 and extends uniformly from either side of the groove 22.

The fountain assembly 13 is supported for application of coating material to the web 14 and includes the support plates 27 adjacent the ends of the tube 16. The support plates 27 are suitably cut out at the top to receive the tube 16 in supporting cradled relation therein. The fountain assembly is mounted for movement about the pivot shaft 28 which extends through suitable bearings 30 mounted on the frame 10. The bracket members 31 are pivotally supported by the bearing 30 and each include an elongated support bar 33 which is rigidly secured to the bracket member 31 and is pivotally secured to the support plates 27 by the pins 35.

The fountain assembly is adjusted with respect to these support bars 33 by adjusting the relative position of the threaded extension 36 of the I-bolt 37 on the bars 33 with respect to the I-bolt 38 secured to the end plates 27. Thus by loosening each of the nuts 41 and 42, the relative position of the bolts 37 and 38 can be varied thereby to change the angular relationship between the fountain assembly 13 and the support bars 33. It should be understood that only one side of the coating apparatus is shown and that the opposite is substantially a mirror image of that shown and described.

An actuator 44 (shown partially in FIG. 1) is used to pivot the bracket 31 about the shaft 30 to move the entire fountain assembly 13 to a position remote from the roll 11 for cleaning and maintenance. The adjustable stop 45 may be provided on the frame to limit the extent that the bracket 31 will move in a counterclockwise direction, as viewed in FIG. 1.

The doctor assembly is an important part of this invention and includes a support assembly which is preferably constructed according to the teachings of the copending United States application of James T. Coghill, Ser. No. 350,910, filed Mar. 10, 1964, now Patent No. 3,301,214, and assigned to the same assignee as this application. That is, the support assembly preferably includes a supporting frame 50 which is rigidly mounted on a cross channel 51 forming a part of the frame 10 of the coating apparatus.

The clamping assembly 52 is mounted for pivotal movement on a shaft 53 by a plurality of arms 54 and is movable with the arms 54 between an advanced or operating position (FIG. 1) and a retracted position (not shown) wherein the clamping assembly 52 is retracted to a position spaced clockwise from the position shown in FIG. 1. The clamping assembly 52 also includes a series of actuators in the form of air motors 55 which are fixedly mounted on support arms 56 and which are connected to the arms 54 by a pin 57 for effecting movement of the assembly 52 between the operating and retracted positions.

The clamping assembly 52 also includes the clamp and pressure bars 59 and 60 which extend transversely at least the width of the web 14 to clamp the support blade 62 in the position shown in FIG. 1, between the pressure bars 59 and 60 and the forward lip 63 extending upwardly from the frame 50. The bar 60 is adjustable along its length as described in the aforesaid Coghill application to vary the force extend along the length of the support blade 62. The entire frame 50 and doctor assembly 15 may be retracted for the purpose of cleaning, maintenance, replacement, and the like, by use of conventional actuators and related equipment, not shown. The overflow trough 65 is provided on the frame 50 below the fountain 13 to collect any coating color which may overflow the fountain during start-up.

FIG. 3 illustrates the self-adjusting doctor assembly 15 including the elongated doctor bar 66 which extends coextensively with the roll 11, and is pivotally supported on the support blade 62. The doctor bar 66 has a flat face 67 with a sloping front wall 68 on the leading portion thereof, while the trailing wall 71 is perpendicular to the flat face 67. The bottom side 72 is also flat and parallel to the face 67, and has a mounting slot 73 therein perpendicular to the flat face 67.

The support blade 62 is coextensive with the doctor bar 66 and has a mounting portion 74 which is rigidly held in position by the clamping assembly 52 between the lip 63 and the pressure bars 59 and 60. The adjustable pressure bar 60 may apply forces to the mounting portion 74 to deform the same along its length for limited adjustment thereof, but this invention may be used without this adjustable feature.

The support blade 62 also includes a pivot portion 76 formed integrally with the mounting portion 74 and disposed at an angle thereto so that this portion is substantially perpendicular to a plane tangent to the adjacent portion of the backing roll 11. The pivot portion 76 is received within the slot 73 in the doctor bar 66, and since the slot has a width substantially wider than the thickness of the pivot portion 76, the doctor bar is free to tilt within limits about the upper edge 77 of the support blade 62. The extent of tilting is limited by the contact between the corners 78 and 79 on the bar 66 at the entrance to the slot 73 so that the bar 66 is free to tilt within limits but cannot be disengaged from the support blade 62 during operation.

In operation, the fountain coater 13 is moved to its operative position with respect to the backing roll 11 and coating is forced from the tube 16 upwardly between the lips 25 and 26 to apply a coating of excess thickness to the web 14 as it moves on the roll 11 past the lips in the manner described in the aforesaid Phelps et al. application. As the web 14 moves with the backing roll 11 past the doctor bar 66, the coating is smoothed and evened to a precise thickness and uniformity. Because of the capability of the doctor bar 66 to pivot about the edge 77 of the support blade 62, the pressure of the wedge of coating liquid between the web 14 and the flat surface 67 automatically evens itself throughout the wedge thereby preventing the occurrence of high pressures which would undesirably force the coating into the substrate of the web 14.

The excess coating on the web 14 is removed as the coating contacts the leading surface 68 of the doctor bar 66, and only the preset thickness is allowed to pass through the tapered wedge which is formed between the fiat surface 67 and the roll 11. The excess coating flows downwardly along the surface 68 and then eventually fiows into the overflow trough 65 and is reprocessed or discarded. The thickness of the coating is dependent upon the space between the surface 67 and the web 14, and this can be readily varied by changing the position of the doctor bar 66 with respect to the web 14 so that the desired thickness of coating is achieved. It will be seen that the thickness of the coating cannot abruptly change across the width due to irregularity in the surface of the backing roll 11, since the pressure within the coating wedge is also uniform in a direction transversely of the roll.

In order to prohibit excessive pressures between the surface 67 and the web and to obtain an effective doctoring action, it is necessary that the pivot point be precisely located to provide for an even distribution of pressure on the flat surface 67 of the doctor bar 66. The pressure distribution on the flat surface is given by the following equation:

u is the viscosity of the liquid between the web and the fiat surface,

V is the velocity of the web,

I is the length of the bearing surface,

h is the distance between the trailing edge and the web or the thickness of the coating, and

K is a function of the point of application of the pressure on the surface:

1 2m+2 1 1 i F I where x is the distance between the trailing edge of the surface and the center of the pressure application,

h and h are the clearances between the flat surface and the traveling web at the leading and at the trailing edge of the surface, respectively. As can be seen, K depends also upon the inclination of the bearing.

The center of pressure of the surface under hydrodynamic pressure is then as follows:

I f Pxdx f Pdx L GWG) 1 hlKpd t) The average pressure P and the friction F on the surface of the doctor bar are important since they affect the penetration of the coating color into the web 14. They can'be calculated by integration or from the following formulas:

where K and K are tabulated functions of the inclination of the surface 67. These values are fairly constant so that the values of K =0.0267 and K =0.614 were chosen, assuming m'=1.

Coating colors are non-Newtonian fluids. They are often considered exhibiting pseudoplastic behavior:

This linear approximation is coarse but suflicient when considering the many different rheological behaviors of coating color. This will permit a good extrapolation of the viscosity at very high shear rates:

n=la log a u=u M (10) A statistical analysis of a wide range of viscosities of coating colors gives a value for a between 0.15 and 0.25 when plotting the decimal logarithm of the viscosity. The value of a=0.20 was used into the calculations, together with the factor M=2.303 to convert decimal into natural logarithms.

The rate of shear S under the bearing and, therefore the viscosity w, are not uniform but the differences are small and the rate of shear at the trailing edge is used in the calculations:

V 18-200 he (11) When the center of the pressure is calculated in accordance with Equations 2 and 5 for a plurality of inclinations of the surface 67, it is found the center of pressure is placed at 40 to 44 percent of the surface width as measured from the trailing edge. To assure the best tiltability, the pivot point must be then placed at this center of pressure, i.e., between about A of the surface from the trailing edge.

The operational viscosity, the pressure and the friction are calculated according to the Equations 10, 6 and 7, for a plurali-.y of different film thicknesses. These calculations are repeated for a plurality of different speeds and again repeated for a plurality of different viscosities. The conclusion reached is that this coater will operate in a medium pressure range. A high solids coating of 10 to 20 pounds corresponds to a film thickness of 1 to 2 mils at a viscosity of 10,000 cps., and a low solid coating of 2 to 4 pounds corresponds to a film thickness of 0.5 to 1 mil at a viscosity of 100 cps. In both cases, the hydrodynamic pressure is calculated to be between 20 and 60 p.s.1.

FIGS. 5 and 6 illustrate another embodiment of the invention wherein a pair of tilting doctor bars and 81 are mounted on the lips 82 and 83 of a fountain coater similar to that shown in FIG. 1. Thus the lips 82 and 83 on the opposite sides of the flow channel 84 have the annular depressions 85 formed in the top surfaces 86 thereof. These lips and depressions extend coextensively with the backing roll 88 and an elongated inflatable tube '90 is disposed in each of the depressions 85. A strip 92 of resilient material, for example, polyurethane foam, is secured in place on the surfaces 86 on the opposite sides of the tubes 90.

The tilting doctor bars 80 and 81 having the curved depressions 93 in the bottom surfaces 95 thereof complementary to the depressions 85 are then placed on top of the tubes 90 and resilient strips 92 so that the top portions of the tubes 90 are received within the depressions 93. The bars 80 and 81 are otherwise very similar to that shown in FIG. 3 except for the manner in which they are mounted for pivotal movement, as explained above. Accordingly, the flat top surface 98, and rear and leading sides 101 and 102, of the downstream bar 81 are substantially identical to those described in connection with the embodiment of FIGS. 1-4. The upstream bar 80 is also similar with the flat top surface 104 having a square side wall 105 on the upstream side and the tapered side wall 106 at the downstream side thereof.

The doctor bars 80 and 81 pivot about the axis of the tube 90 in response to the pressures created on the surfaces 98 and 104 thereof during the coating operation. Since the pivot axis [of these tubes is disposed in accordance with the calculations set forth above, the bar 80 or 81 is tilted until the pressure within the coating wedge is uniform. The tilting movement of the bars 80 and 81 causes one of the resilient pads 92 to be compressed without any substantial resistance to the tilting movement of the bars, and when this tilting force is removed the resilient pads return the bars to their horizontal position.

In operation of this embodiment, the coating fluid under pressure is forced through the tube 107 and upwardly into the channel 84 formed between the vertical lips 82 and 83. Excessive coating is applied under pressure to the moving web 108 by the flow between the tilting doctor bars 80 and 81 and, as the web moves over one of the doctor bars 80 or 81, a smoothing operation substantially identical to that described above occurs. That is, the pressures created between the web 108 and the smooth flat surface 98 or 104 of the doctor bar 80 or 81 causes the doctor bar to tilt to a position wherein the coating is evenly smoothed without patterns and of uniform thickness.

If the web is moving to the right, as viewed in FIG. 5, the upstream doctor bar 80 on the left lip 82 merely serves to limit or confine the flow of liquid between this doctor bar 80 and the backing roll 88. With this arrangement, however, the direction of the roll 88 and web 108 can be reversed without any adjustment of the doctor bar 80 or 81 since the bar 80 then operates in the same manner as the bar 81, while the bar 81 then operates as bar 80.

FIGS. 7 and 8 disclose a further embodiment of this invention wherein the tilting doctor bar 110 is used in a reverse roll coater 111. In this arrangement, the lower roll 112 rotates through a reservoir 113 of coating liquid which thus causes an excessive amount to adhere temporarily to the outer surfaces 114 thereof during rotation. The web 116 to be coated moves with the backing roll 117 in a direction opposite to the adjacent portion of the lower roll 112 which is spaced from the web a slight distance so that the coating on the roll 112 is applied to the web 116 in a known manner.

The tilting doctor bar 110 is used to even the coating on the lower roll 112 prior to the transfer of this coating to the web 116. The tilting doctor bar 110 is pivotally supported on a pivot member 120 for cooperation with the surface 114 of the lower roll 112 to provide a smooth pattern free coating thereon of preset thickness in the manner similar to that described hereinbefore in connection with FIGS. 1-4.

A further embodiment of the invention is illustrated schematically in FIG. 9 wherein both sides of a moving web 125 are coated and thus no backing surfaces per se are used. Specifically, a pair of fountain coaters 126 and 127 apply an excessive coating to the opposite sides 132 and 133 of the moving web 125. Downstream from these fountain coaters is a pair of tilting doctor bars 134 and 135 pivotally supported on the pivot members 136 and 137 and disposed on opposite sides of the moving Web to remove the excess liquid and to smooth this coating in the manner described above. In this embodiment, the doctor bars 134 and 135 also work together forming backing surface for the opposite doctor bar, and the coating operation is otherwise substantially identical to that described above. Moreover, the pivot axes of the doctor bars 110, 134, and 135 are selected in accordance with the calculations set forth in connection with FIGS. l-4.

While each of the embodiments of the invention describes the surface adjacent the moving web or backing surface as a flat face or smooth surface, these terms are intended to include surfaces which are rounded either in a concave or convex manner, or which have groove-s disposed therein either longitudinally of the face or transversely thereof to facilitate the application of the coating. The important feature of the invention is that the surface is free to tilt about an axis in accordance with the pressures developed within the wedge coating material formed between the surface being coated and the face so that this pressure is uniform throughout the wedge thereby facilitating high speed coating application without touching the surface being coated. The uniform pressures are less than those that would otherwise normally be present, and therefore the coating fluid is not forced into the substrate of the web, or through the web itself as would occur if higher pressures were utilized.

What is claimed is:

1. Apparatus for doctoring a coating liquid which has been applied to a smooth surface or the like, comprising an elongated doctor bar, support means tiltably supporting said doctor bar in a preset position for pivotal movement about an axis parallel to the smooth surface, means for effecting relative movement between said smooth surface and said doctor bar, said doctor bar having a flat face adjacent the smooth surface with leading and trailing edges thereon, a pivot connection between said doctor bar and said support means, said connection being parallel to said flat face and located a distance which equals about 40 to 44% of the width of said flat face measured from said trailing edge, and means for moving said smooth surface with respect to said doctor bar to enable said doctor bar to be pivoted automatically to a doctoring position wherein the pressure of the coating liquid in the space between said smooth surface and said flat face is uniform and a smooth coating of precise thickness is created on the smooth surface.

2. Apparatus for applying a wet coating to a web. or the like supported by a backing surface, comprising means for applying excessive coating liquid to the web, means for doctoring said excessive coating including an elongated doctor bar, support means tiltably supporting said doctor bar in a preset position with respect to said backing surface for pivotal movement about an axis parallel to the web, means for effecting movement of said web with respect to said smooth surface on said doctor bar, said doctor bar having a smooth face adjacent the web surface with leading and trailing edges thereon, and a pivot connection between said doctor bar and said support means located a distance which equals about 40 to 44% of the width of said fiat face measured from said trailing edge, said doctor bar being adapted to pivot automatically to a position wherein the pressure of the coating liquid in the space between said smooth surface and said web is uniform throughout said space to produce a smooth pattern-free coating of precise thickness on the web.

3. Apparatus for coating a smooth surface or the like, comprising means for applying excessive coating liquid to the smooth surface, means for doctoring said excessive coating including an elongated doctor bar, said doctor bar having a flat face adjacent the smooth surface with leading and trailing edges thereon, means defining a slot in a surface of said doctor bar parallel to and opposite said flat face, support blade means coextensive with said doctor bar and having a thickness less than the Width of said slot so that a portion of said support blade means is received in said slot to support said doctor bar for pivotal movement about an axis parallel to the smooh surface, said slot being located a distance measured from said trailing edge which equals about 40 to 44% of the width of said flat face, and means for effecting relative movement between said smooth surface and said doctor bar, said doctor bar being adapted to be tilted automatically to a position wherein the pressure of the coating liquid in the space between said smooth surface and said fiat face is uniform and a smooth coating of precise thickness is created on the smooth surface.

4. Apparatus for coating a smooth surface or the like, comprising means for applying excessive coating liquid to the smooth surface including a pair of parallel lips defining therebetween a flow path, means for forcing coating liquid through said flow path to apply the excessive coating to the smooth surface, one of said lips having an elongated doctor bar pivotally supported for movement of said doctor bar about an axis parallel to the smooth surface, said doctor bar having a flat face adjacent the smooth sunface, and means for effecting relative movement between said smooth surface and said doctor bar, said doctor bar being adapted to pivot automatically to a position wherein the pressure of the coating fluid in the space between said smooth surface and said fiat face is uniform and a smooth coating of precise thickness is created on the smooth surface.

5. Apparatus for coating a smooth surface or the like, comprising means for applying excessive coating liquid to the smooth surface including a pair of parallel lips defining therebetween a flow path, means for forcing coating liquid through said flow path to apply the excessive coating to the smooth surface, each of said lips having an elongated doctor bar pivotally supported thereon for pivotal movement of said doctor bar about an axis parallel to the smooth surface, said doctor bars having a. flat face adjacent the smooth surface, and means for effecting relative movement between said smooth surface and said doctor bars, said doctor bars being adapted to pivot automatically to a position wherein the pressure of the coating liquid in the space between said smooth surface and said flat face is uniform and a smooth coating of precise thickness is created on the smooth surface.

6. Apparatus for coating a smooth surface or the like, comprising means for applying excessive coating liquid to the smooth surface including a pair of parallel lips defining therebetween a flow path, means for forcing coating liquid through said flow path to apply the excessive coating to the smooth surface, one of said lips having a fiat outer surface having an elongated groove therein, a pivot tube having a portion which extends into said groove, an elongated doctor bar having a groove in the back portion thereof for receiving a portion of said pivot tube for pivotal movement of said doctor bar about the axis of said tube, said doctor bar having a fiat face adjacent the smooth surface, and means for effecting relative movement between said smooth surface and said doctor bar, said doctor bar being adapted to be moved automatically to a position wherein the pressure of the coating liquid in the space between said smooth surface and said flat face is uniform and a smooth coating of precise thickness is created on the smooth surface.

7. Apparatus as defined in claim 6' wherein resilient means are provided on opposite sides of said pivot tube between said outer surface and said doctor bar for urging said doctor bar to a central position.

8. In a reverse roll coater for applying a liquid coating to a moving web, the improvement of apparatus for doctoring the excessive liquid coating applied to the coating roll as it rotates with its lower portion submerged in liquid coating, said improvement comprising an elongated doctor bar at least equal to the width of the web, support means tiltably supporting said doctor bar in a preset spaced relation with respect to the coating roll for pivotal movement about an axis parallel to the axis of the coating roll, said doctor bar having a smooth face adjacent and spaced from the coating roll, and said doctor bar being adapted to pivot to a position wherein the pressure of the coating liquid in the space between the roll and said flat surface is uniform and creates a smooth coating of precise thickness on the web at high speed.

9. Apparatus for coating a web of paper or the like, comprising means for applying excessive coating liquid to the opposite sides of the web, a pair of elongated doctor bars disposed on opposite sides of the web to provide a backing support for each other, support means tiltably mounting said doctor bars in a preset position for pivotal movement about an axis parallel to the plane of the web, each said doctor bar having a smooth face adjacent the web, and means for effecting relative movement between the web and said doctor bars, said doctor bars being adapted to pivot to a position wherein the pressure of the coating liquid in each of the spaces between the web and said smooth faces is uniform and creates a smooth coating of precise thickness on the web at high speed.

10. Apparatus for coating a web, comprising means for applying a coating liquid to the opposite sides of the web moving at high speed, a pair of elongated doctor bars disposed on opposite sides of the web to provide a backing support for each other, support means tiltably mounting said doctor bars in a preset position for pivotal movement about an axis parallel to the plane of the web, and each said doctor bar having a smooth face adjacent the web, said doctor bars being adapted to pivot automatically to a position wherein the pressure of the coating liquid in each of the spaces between the web and smooth faces is uniform and creates a smooth coating of precise thickness on the web at high speed.

11. A high speed process of doctoring an excessive coating on a web moving at high speed, comprising the steps of positioning a free tilting doctor bar having 8. smooth face adjacent but spaced from the web, effecting relative movement between said web and said smooth face, supporting said web on the side opposite said doctor bar and directly opposite said smooth face so that the coating forms an elongated tapered wedge in 'the space between said smooth face and the web, and automatically tilting said doctor bar to a position wherein the pressure of said coating within said wedge is constant and uniform throughout thereby enabling lower pressures within said Wedge, said smooth face cooperating with the coating and web to even the coating and to remove excess coating so that a coating of precise depth is created.

12. A high speed process of coating on a smooth surface with a liquid coating, comprising the steps of forcing the coating liquid upward between the two lips which are closely spaced from said smooth surface to apply excessive coating liquid to said smooth surface, effecting relative movement between said lips and said smooth surface, said lip on the downstream side of said passage having a free tilting doctor bar including a flat face disposed adjacent but spaced slightly from the smooth surface, said relative movement forming an elongated tapered wedge in the space betwen said flat face and the smooth surface, and automatically tilting said doctor bar to a position wherein the pressure of said coating within said wedge is constant and uniform throughout, said flat face cooperating with the coating and smooth surface to even the coating and to remove excess coating so that a coating of precise thickness is provided on said smooth surface.

13. A high speed process of simultaneously doctoring a coating on the opposite side of a web moving at high speed, comprising the step of positioning a free tilting doctor bar having a smooth face adjacent but spaced slightly from the opposite sides of the web, effecting rela tive movement between said web and said smooth faces, supporting said web between said smooth faces so that the coating forms a pair of elongated tapered wedges in the spaces between said smooth faces and the web, and automatically tilting said doctor bars to a position wherein the pressures of said coating within said wedges are constant and uniform throughout thereby enabling lower pressures within said wedges, said smooth faces cooperating with the coating and the web to even the coating and provide a coating of precise depth.

References Cited UNITED STATES PATENTS 8/1941 Berch et al. 118 410 12/1956 Williamson et al. 118-126 

